1. Field of the Invention
This invention relates to a measurement-while-drilling (MWD) nuclear magnetic resonance (NMR) tool, and more particularly to a nuclear magnetic resonance tool having a non-rotating sleeve for containing a portion of the NMR electromagnetic transmitters, sensors and associated electronics which isolates the NMR sensor and transmitter from lateral motion induced by the drill string during drilling operations.
2. Description of the Related Art
To obtain hydrocarbons such as oil and gas, a drilling assembly (also referred to as the xe2x80x9cbottom hole assemblyxe2x80x9d or the xe2x80x9cBHAxe2x80x9d) carrying a drill bit at its bottom end is conveyed into the wellbore or borehole. The drilling assembly is usually conveyed into the wellbore by a coiled-tubing or a drill pipe. In the case of the coiled-tubing, the drill bit is rotated by a drilling motor or xe2x80x9cmud motorxe2x80x9d which provides rotational force when a drilling fluid is pumped from the surface into the coiled-tubing. In the case of the drill pipe, it is rotated by a power source (usually an electric motor) at the surface, which rotates the drill pipe and thus the drill bit.
Bottom hole assemblies (xe2x80x9cBHAxe2x80x9d) generally include several formation evaluation sensors for determining various parameters of the formation surrounding the BHA during the drilling of the wellbore. Such sensors are usually referred to as the measurement-while-drilling (xe2x80x9cMWDxe2x80x9d) sensors. Such sensors traditionally have electromagnetic propagation sensors for measuring the resistivity, dielectric constant, or water saturation of the formation, nuclear sensors for determining the porosity of the formation and acoustic sensors to determine the formation acoustic velocity and porosity. Other downhole sensors that have been used include sensors for determining the formation density and permeability. The bottom hole assemblies also include devices to determine the BHA inclination and azimuth, pressure sensors, temperature sensors, gamma ray devices, and devices that aid in orienting the drill bit in a particular direction and to change the drilling direction. Acoustic and resistivity devices have been proposed for determining bed boundaries around and in some cases in front of the drill bit. More recently, nuclear magnetic resonance (xe2x80x9cNMRxe2x80x9d) sensors have gained extreme interest as MWD sensors as such sensors can provide direct measurement for water saturation porosity and indirect measurements for permeability and other formation parameters of interest.
NMR sensors utilize permanent magnets to generate a static magnetic field in the formation surrounding the MWD tool. A radio frequency (RF) coil disposed between the magnets or around the magnets induces a RF magnetic field. The magnets and the RF coils are positioned so that the static and RF fields are perpendicular to each other at least over a portion of the formation surrounding the NMR tool wherein the Lamor frequency which is determined by static field has a substantially uniform strength. This region is the region of interest or region of investigation. The NMR measurements corresponding to such region are needed to determine the formation parameters of interest.
MWD sensors are located inside or outside of a drill collar for performing measurements on the formation and its fluid content. A conventional drill collar is a metallic structure that conveys the torque required for the drilling operation. The drill collar acts as a conduit for the drilling fluid or mud that is used to lubricate the drill bit and carry the cuttings to the surface. Since audio and radio frequency electromagnetic fields do not penetrate the metallic body of the drill collar, sensors of electromagnetic fields are mounted outside the metallic body of the drill collar. These sensors are subject to abrasions resulting from particles in the drilling mud and the impact of the sensor against the earth formation. In some cases, shields or protective coatings are used on the drill collar to protect the sensors. Often, wear bands are employed on the drill collar to provide an appropriate standoff distance between the sensors and the formation thereby reducing or eliminating the impact of the sensor physically contacting earth formation.
A measurement-while-drilling tool is described in EP-A-0581666 (Kleinberg). The tool comprises a tubular drill collar; a drill head positioned at an axial end of the drill collar; and an NMR sensor. The NMR sensor comprises a pair of tubular main magnets (which generate a static (B0) magnetic field) each located in an internal recess of the drill collar, and an RF antenna located in an external recess in the drill collar between the main magnets. The RF antenna recess is optionally filled with a magnetically soft ferrite to improve the efficiency of the antenna.
An NMR well logging system is described in U.S. Pat. No. 4,629,986 (Clow et al.). Each of a pair of main magnets are separated by a gap in which a solenoid RF antenna is symmetrically disposed. The solenoid has a core of high permeability ferromagnetic material (soft ferrite).
The ferrite members may be axially spaced and/or spaced at right angles to the axis of the tool. A primary consideration in the design of an NMR MWD tool is making the NMR measurement insensitive to the effect of lateral tool motions, such as vibration and whirl. To a first approximation it is clear that it will not be possible to re-focus the NMR signal in the sensitive region if the tool is displaced laterally (i.e. in a direction parallel to the radius) during the pulse sequence by a distance which comprises a significant portion of the radial thickness of the sensitive region. Little is known about the precise motions of drilling tools down hole, however, the typical range of displacement is from 1 to 10 mm at frequencies of a few Hz. Rotation periods are between 1 and 3 Hz. The typical NMR measurement lasts from 50 ms to 1 sec, thus, these motions provide a significant detrimental effect on NMR measurement accurancy. Thus there is a need NMR tool configuration designed for reducing the effects of lateral motion on a NMR sensor during drilling operations.
The disadvantages of the prior art are overcome by the apparatus and method of the present invention. The present invention isolates the MWD NMR sensor or at least the motion sensitive part of the NMR tool from the lateral motion of the drill string while drilling a borehole. In one aspect of the present invention a NMR device is presented that includes components on a drill collar having a non-rotating sleeve containing permanent magnets for generating a static magnetic field, B0 for NMR measurements. The non-rotating sleeve is intermittently clamped against a borehole wall during NMR measurements and thus decoupled from the drill collar movements and drilling vibrations during NMR measurements. The NMR RF transmitter and receiver are located on the rotating drill collar. In another aspect of the present invention, the permanent magnets and the RF receiver antenna and electronics are located on the non-rotating sleeve which is intermittently clamped against the borehole wall during NMR measurements and thus decoupled from the drill collar and drilling vibrations during NMR measurements. Power for the receiver antenna can be derived from transmitter power or a separate transformer. The transmitting antenna is located on the rotating drill collar. In another aspect of the present invention at least one non-rotating stabilizer is provided above or below the NMR sensor located on the drill collar. The stabilizer is activated to stabilize the rotating NMR sensor located on the drill collar in the bore hole. In yet another aspect of the present invention the permanent magnets and receiving and transmitting antennae are located on a non-rotating sleeve that is intermittently clamped against the borehole wall during NMR measurements to decouple the permanent magnets and receiving and transmitting antennas from drilling vibrations during NMR measurments. The transmitter electronics and other electronics are located on the rotating drill collar. A rotating transformer transmits RF power to the transmitting antenna.